1. Field of the Invention
The present invention relates to a diversity receiving apparatus and, more particularly, to a diversity receiving apparatus which can realize a low electric power consumption.
2. Description of the Related Art
In a conventional diversity receiving apparatus, electric power is supplied to a plurality of receivers, thereby making them operative.
The diversity receiving apparatus is a receiving apparatus for extracting a detected output or demodulated output of a receiver exhibiting a good receiving state from signals received by a plurality of receivers, such that the outputs of all but one of the plurality of receivers are unnecessary. A conventional example of a diversity receiving apparatus using two receivers will now be described. FIG. 13 shows a construction of a diversity receiving apparatus comprising a receiver 1A and a receiver 1B. After received desired signals are obtained by detectors 2A and 2B, they are demodulated by demodulators 3A and 3B. A level comparator 4 compares the received signal levels of the received desired signals obtained by the detectors 2A and 2B. From the comparison result, the demodulated signal having a higher received signal level is selected by a selector 5 and is output as a final demodulated output. FIG. 14 shows a relation between the received signal levels in the two receivers 1A and 1B. The abscissa denotes time and the ordinate indicates a received signal level of the received desired signal after completion of the detection. The received signal level ordinarily changes with the elapse of time due to fading or the like. For example, when antennas of two receivers are mounted at positions having a small correlation therebetween and signals are received, the received signal levels tend to vary over time shown in, for instance, FIG. 14. In the diversity receiving apparatus shown in FIG. 13, a received signal level of the receiver 1A and a received signal level of the receiver 1B are always compared by the level comparator 4. The demodulated signal having the higher received signal level is selected by the selector 5. Therefore, the demodulated signal of the receiver 1A is selected in time areas 1 and 3 in FIG. 14, while the demodulated signal of the receiver 1B is selected in time area 2. The demodulated signal having the higher received signal level, consequently, can be always obtained. In the conventional diversity receiving apparatus shown in FIG. 13, however, since the two receivers are always in the operating state, the total electric power consumption of the two receivers is twice as large as the electric power consumption in the case of using one receiver.
As a method of reducing the electric power consumption by suppressing a deterioration in receiving characteristics of the diversity receiving apparatus, for example, the conventional method disclosed in Japanese Patent Application No. 1-94025 (published as Japanese Kokai 2-271724) can be mentioned. The method of reducing the electric power consumption in such a conventional example will now be described with reference to FIG. 15. According to the conventional method shown in FIG. 15, two thresholds (threshold 1&gt;threshold 2) are provided for the received signal levels of the receivers 1A and 1B. When either one of the received signal levels of the receivers 1A and 1B is equal to or larger than the threshold 1 and the other one is equal to or less than the threshold 2, only the receiver whose received signal level is equal to or larger than the threshold 1 is made operative. That is, in the area 1 in FIG. 15, since the received signal level of the receiver 1A exceeds the threshold 1 and the received signal level of the receiver 1B is equal to or less than the threshold 2, only the receiver 1A is made operative and the power supply to the receiver 1B is stopped. In the area 2, since both of the received signal levels of the receivers 1A and 1B are equal to or less than the threshold 1, both of the two receivers 1A and 1B are made operative and the received signal levels of the received desired signals obtained by the detectors 2A and 2B are compared by the level comparator 4. The demodulated signal having the higher received power level is selected by the selector 5. In the area 3, on the other hand, since the received signal level of the receiver 1B exceeds the threshold 1 and the received signal level of the receiver 1A is equal to or less than the threshold 2, only the receiver 1B is made operative and the power supply to the receiver 1A is stopped. In the area 4, since both of the received signal levels of the receivers 1A and 1B are equal to or less than the threshold 1, both of the two receivers 1A and 1B are made operative. In the area 5, only the receiver 1A is made operative in a manner similar to the case of the area 1. Thus, only either one of the receivers 1A and 1B operates in the areas 1, 3, or 5, so that there is an effect of reduction of the electric power consumption for a period of time corresponding to such areas. In the conventional method shown in FIG. 15, however, since the two receivers always operate in the areas 2 and 4, for example, in the case where a frequency at which the received signal levels are relatively low and the received signal levels of the two receivers exceed the threshold 1 is low, or in the case where a frequency at which the receiving states are relatively good and the received signal levels are lower than the threshold 2 is low, a frequency at which the two receivers operate simultaneously increases, so that the effect of the reduction of the electric power consumption decreases.
A conventional example of a diversity receiving apparatus which is used in a digital communication system using a time division multiple access system (TDMA system) will now be described.
In the digital communication system using the time division multiple access system, information is intermittently communicated at a preset time period. As shown in FIG. 16, namely, a time base is divided into frames, thereby constructing a time division received signal 10. Each frame is divided into three slots 1, 2, and which are assigned to respective users and time compression is performed for every slot. For example, when the slot 2 is assigned, the reception is performed for only the period of time of the slot 2 and the reception is stopped for the other periods of time (slots 1 and 3). The time compressed received signal is demodulated and, after that, it is expanded in time, so that an expanded received signal 11 is derived.
In the diversity receiving apparatus in the digital communication system using the TDMA system mentioned above, for instance, when the conventional method shown in FIG. 14 is applied, the two receivers 1A and 1B are made operative for the entire periods of time of all of the slots 2. Even in case of applying the conventional method shown in FIG. 15, the two receivers 1A and 1B have to be made operative for the entire periods of time of all of the slots 2 in the areas 2 and 4.
Generally, in addition to a level difference, a phase difference occurs between the detected signals or demodulated signals obtained by a plurality of receivers. Since there is a phase difference in the digital receiving system, even if a plurality of detected signals are simply switched and coupled to a demodulator with respect to time, there is a possibility that the data cannot be demodulated. Therefore, a method whereby a plurality of receivers up to the demodulators are provided and are continuously operated and one of a plurality of demodulated signals is switched by a selector; or a method whereby the plurality of demodulated signals are not switched for the slot periods of time but are switched on a slot unit basis, is used. An example of the latter method is shown in a timing chart of FIG. 16. The received signal levels are compared in the one frame, and the receiver to be selected in the next frame is decided on the comparison. In this case, it is necessary to make two receivers operative in the previous frame for to enable selection of a receiver in the present frame. Since the receiver in the present frame is selected based on the comparison in the previous frame, there is a problem such that in the case where a fluctuation in received signal level is shorter than the frame interval with respect to time, the correct receiver cannot be selected.
As a method of solving such a problem, as shown in FIG. 17, there is a method whereby a part of the signals of the slot 1 just before the slot 2 to be received is received by two receivers and their received signal levels are compared and only one of the receivers is made operative in accordance with the result of the level comparison of the slot 1 just before the slot 2 (Journal of the Institute of Electronics, Information and Communication Engineers of Japan, Vol. 73, No. 8, August 1990, pp. 812-818). According to such a method, since the levels are compared in a state near the received signal level of the slot to be received, the proper receiver can be selected so long as a communication system in which the slot 1 just before the slot 2 to be received certainly exists. However, in a system in which the slot 1 just before the slot 2 to be received is not always used, there is a problem that the proper receiver cannot be selected.